(WASHINGTON) -- President Donald Trump escalated his attacks on mail-in voting on Wednesday, targeting election officials in two battleground states over efforts to expand access to vote-by-mail ahead of November.

The president, who has often railed against mail voting by alleging -- without evidence -- that the voting alternative is ripe for fraud, singled out Michigan and Nevada in the morning over each states' effort to use absentee and mail-in ballots for upcoming elections over the coming months by falsely claiming the states acted "illegally" and threatening to "hold up" federal funding to those states.

In the afternoon, Trump defended his criticism of Michigan and didn't say what funding might be withheld, but also indicated that it may not be necessary.

Trump's latest aspersion on vote-by-mail comes against the backdrop of dozens of states abruptly altering their election playbooks to safely run contests during the coronavirus pandemic. The president has repeatedly criticized mail voting, a primary alternative to in-person voting, and casted doubt on the integrity of the practice during the crisis.

In a pair of tweets Wednesday morning, Trump first falsely claimed that Michigan is mailing all 7.7 million registered voters absentee ballots for this year's down-ballot primaries and general elections, a mischaracterization of the state's absentee ballot effort. Earlier this week, Michigan Secretary of State Jocelyn Benson announced the state intends to mail absentee ballot applications, not ballots, to registered voters.

"Michigan sends absentee ballots to 7.7 million people ahead of Primaries and the General Eection," Trump wrote. "This was done illegally and without authorization by a rogue Secretary of State. I will ask to hold up funding to Michigan if they want to go down this Voter Fraud path!"

The president later deleted the original tweet, and posted a new one correcting his earlier one by adding the word "applications."

A spokesperson for Secretary of State Jocelyn Benson called Trump's tweet "false," saying in a statement, "The Bureau of Elections is mailing absent voter applications, not ballots. Applications are mailed nearly every election cycle by both major parties and countless advocacy and nonpartisan organizations. Just like them, we have full authority to mail applications to ensure voters know they have the right to vote safely by mail."

Of the 7.7 million registered voters in the state, about 1.3 million are on a permanent absent voter list -- meaning those voters are mailed applications before every election by local election clerks, according to a release from secretary of state.

Benson further responded directly to the president on Twitter, pointing to her Republican counterparts in other states that took similar actions with vote-by-mail.

"Hi! I also have a name, it's Jocelyn Benson. And we sent applications, not ballots. Just like my GOP colleagues in Iowa, Georgia, Nebraska and West Virginia," she tweeted.

Shortly after his tweet about Michigan, the president put Nevada in his crosshairs, a state in which all active registered voters will automatically receive a mail-in ballot for the state's upcoming June primary.

The secretary of state's office refuted Trump's assault on the state's mail-in election in a statement Wednesday afternoon.

"Secretary Cegavske lawfully declared the 2020 primary election as a mail-in election," the statement says. "Nevada has many safeguards in place to ensure the integrity of an all-mail election, including signature requirements and verification processes, preprinted ballot return envelopes, barcode tracking, and laws against ballot harvesting. Voters concerned with mailing in their ballot may drop off their ballot at any designated drop-off location in their county."

In both tweets, the president also called the moves illegal and issued a stern threat against state leaders, suggesting he will retaliate by "holding up" funds to the state.

White House press secretary Kayleigh McEnany declined to comment on what Trump thought was illegal about actions taken by officials in those states, referring questions to Trump's re-election campaign, which in turn referred questions to the Republican National Committee.

When asked to detail what is illegal about Michigan sending out absentee ballot applications, as the president claimed in a tweet on Wednesday, RNC national press secretary Mandi Merritt said, "We have been clear that we cannot have rogue state officials or activist courts making unilateral decisions. We continue to support lawful absentee voting with the proper safeguards in place, safeguards which Democrats are suing to eliminate in states like Michigan."

The decision by Secretary of State Barbara Cegavske, a Republican, to move to a predominantly mail election in Nevada cleared a significant hurdle recently, after a federal judge ruled against a conservative group that sought to block the nearly all-mail election. The judge rejected the claims made by the group, True the Vote, which alleged that "voter fraud" would plague the mail voting in the state, writing in the ruling, "their claim of voter fraud is without any factual basis."

Nevada has been at the center of a legal scuffle between Democrats and Republicans over efforts to expand vote-by-mail, attracting attention from both national parties over the issue.

After Cegavske shifted the state's June 9 down-ballot primary to all-mail, Democrats filed a lawsuit to expand voting access, including mail voting, by allowing all registered voters to be mailed ballots, not just to those with "active" status, increasing polling sites in counties across the state to prevent long lines, and suspending the state's voter assistance ban to allow more voters to seek out help in the collection and delivery of ballots.

National and state Republicans intervened on the suit, alleging that it will allow for "ballot harvesting" and "eliminate ballot integrity."

The GOP has committed $20 million to "protect election integrity," which is double the initial commitment of $10 million from February. The increase in the legal budget comes as dozens of legal challenges emerge after a number of states abandoned in-person voting and sought other abrupt changes to elections during the coronavirus.

Unlike Nevada, Michigan has not been the subject of a major lawsuit over mail voting yet, but the move by Benson could face a legal challenge from Republican state lawmakers, who are currently at sharp odds with Michigan's Democratic Gov. Gretchen Whitmer over her emergency declarations. GOP lawmakers even brought a lawsuit against Whitmer seeking to limit her emergency powers. Trump, too, has clashed with Whitmer during the coronavirus crisis over federal assistance.

The latest dispute over vote-by-mail comes two years after Michigan eased restrictions on voting practices, by allowing any voter to cast an absentee ballot -- without an excuse. The change led to a nearly 30% increase in Democratic primary participation for the March 10 presidential primary contest, compared to the last cycle, according to the state party.

It's also unclear what federal funding Trump would -- or could -- hold up to Michigan or Nevada, as he threatened. Michigan has already drawn down $11.5 million in federal funding from the CARES Act for election assistance and is spending $4.5 million of that total on this effort to send applications to registered voters.

The president didn't elaborate on his threat, but in his tweets, he tagged the U.S. Treasury Department and the acting director of the White House's Office of Management and Budget, Russell Vought.

The White House press secretary declined to comment on what funds he was referring to.

"I won't get into exactly what the funding considerations are," she told reporters at the White House Wednesday.

McEnany also said Trump's "tweets were meant to alert" Vought and Treasury Secretary Steven Mnuchin "about his concerns with trillions of dollars going to the state."

The Treasury Department did not respond to a question about what funds Trump wanted held up or if it was taking any action to hold up funding.

A spokesperson for the Office of Management of Budget would likewise not comment, but shared a statement from an unnamed senior administration official that read, "No decisions have been made at this time, discussions are on-going."

As Trump heads to Michigan on Thursday for a visit to a Ford Motor plant in Ypsilanti, the state's chief executive called Trump's claims inaccurate and "disheartening."

"To see Twitter this morning and to see rhetoric like that is disheartening because I think at first it shows you that there was a lack of understanding what the secretary of state was doing," Whitmer said. "We've got to take politics out of this crisis moment and remember we're all Americans. We are all fighting for our lives here and for our economy."

Trump has often claimed that vote-by-mail is "corrupt" and could lead to election fraud. But experts dismiss the notion that voter fraud is an issue with mail voting, while also arguing that mail voting does not decisively give one party's camp an edge over the other.

"There is no widespread fraud in mail voting," Dr. Lorraine Minnite, a political science professor at Rutgers University and author of The Myth of Voter Fraud, told ABC News.

"It's not really clear which party benefits by putting a mail ballot request step into the voting process," said Michael McDonald, an elections expert and political science professor at the University of Florida. "You look at what happened in Wisconsin, for example ... there was a much lower rate of mail balloting in rural jurisdictions."

Even Trump requested a mail ballot for Florida's Republican primary in March and has voted absentee in past cycles.

But the Republican Party isn't exactly echoing Trump's broadside on mail voting.

On Monday, RNC Chairwoman Ronna McDaniel told reporters, "Personally, I don't really have an issue with absentee ballot request forms being sent out to voters, as much as ballot being sent directly to voters, I think the request form is one mechanism of ensuring that that voter is who they say they are, as long as you keep those signature verification laws in place."